Method of developing a negative electrostatic latent image



United States Patent 3,220,830 METHOD OF DEVELOPING A NEGATIVE ELECTROSTATIC LATENT IMAGE Thomas T. Kashiwabara, Fort Lee, N.J., assignor to Keuifel & Esser Company, Hoboken, N.J., a corporation of New Jersey No Drawing. Filed Oct. 2, 1961, Ser. No. 142,022 8 Claims. (Cl. 961) The present invention relates to electrophotography and more particularly to the development of the latent image on an electrophotographic material by a liquid developer.

Although liquid developers have been used for developing electrostatic latent images, the previously known processes and the liquids used have not been suitable for gen: eral use due to infiammability and toxic eilects thereof with resultant danger from fires and danger to the health of the operators.

One object of the present invention is to provide an electrophotographic method of forming latent images on a conductive base including metallic bases, paper bases and the like and to develop the latent electrostatic image by means of a liquid developer which is usable under widely varying conditions and which is safe to use from the standpoint of fire hazard and from the standpoint of health of the operators.

Another object of the invention is to provide a practical and rapid method of forming a pattern image on an electrophotographic surface such as a portion of an airplane, automobile part or template coated with an electrophotographic layer.

Another object is to provide an electrophotographic coating which is readily applicable to a conductive surface where an image is desired and which provides uniform electrophotographic properties to said surface.

Another object is to provide a liquid developer having toner particles which yield high contrast and which produce line images of fine resolution.

Briefly a conductive base support such as metal, treated paper or the like is provided with an electrophotographic coating which accepts and retains an electrostatic charge. The electrostatically charged coating is subjected to a light pattern image which discharges the light struck areas and leaves an electrostatic charge image in the unexposed areas. The electrostatic charge image is developed by the deposition of a toner from a halogenated hydrocarbon liquid. The image may be fixed in a suitable manner including solvent fixing and thermal fixing. The fixed image on its support can be used for a final copy or used as a I printing plate after proper treatment for printing copies by accepted printing methods.

Example 1 A metal template was made by coating a metal base of cleaned aluminum or magnesium plates with a template lacquer of substantially the following composition:

Grams 40% Acryloid B72 (polymer of esters of acrylic and methacrylic acids) in toluene 200 Xylene 180 Zinc oxide (French Process Green Seal No. 8) 240 Titanox RA (titanium dioxide) 80 Total 700 This template lacquer was applied by spraying, pourdown, whirling, roller coating or dip coating to obtain a smooth coating of 0.8 to 1.0 mil when dry. For spraying application the lacquer was diluted with xylene to a viscosity of 55 to 65 seconds using a viscosity cup of the efilux type used to measure the time required for 50 milli- 3,220,830 Patented Nov. 30, 1965 Grams Acryloid B-72 (40% in toluene) 400 Xylene 358 Zinc oxide (Green Seal No. 8) 480 Titanium dioxide 160 This mixture was ground in a pebble mill at 75 to rpm. until the particle size was reduced to one mil or less. The grind time was about 3 hours. The grind data follows:

Pigment to binder ratio 4:1. Zinc oxide to binder ratio 3:1. Pigment volume concentration (volume of pigment divided by total volume of solids multiplied by 100) 47.7% Density 1.47 grams/cc. at

78 F. Particle size 1 mil or less.

100 cc. of the above grind were diluted to cc. with xylene to a viscosity of 40 to 45 seconds on the viscosity cup as before. The lacquer was sprayed onto a support at 35 to 40 pounds per square inch pressure in a well ventilated hood or room. It was found that two passes of the spray were generally suificient to give a dry film thickness of 0.7 to 1.0 mil. Care was exercised to prevent excessive evaporation of the solvent during passage through the air to prevent powdering and to prevent sagging in the coating due to excessively thick coatings. The coating was allowed to air dry at room temperatures to remove any residual solvent. It was also found that drying at F. for five minutes would also remove the residual solvent.

The coated conductive support was charged by exposing the coating to a negative corona discharge for 2 to 5 seconds at 6000 to 8000 volts with the conductive support grounded. Care was taken to avoid overcharging which caused objectionable background and under-charging which caused flashing or streaks.

The charged coating was exposed through a master to a 100 watt tungsten filament clear bulb at 16 inches for five to ten seconds. The tungsten filament lamp was found to be satisfactory but an ultra violet lamp was found to be more efiicient since the zinc oxide is more responsive in the ultra violet region of the spectrum. The images were produced both by projection and by contact printing.

After exposing the coating to light the coated support was placed in a tank containing a liquid developer of the following formula to obtain a positive image of the diapositive transparency:

40% Amberol M-82, a phenol formaldehyde rosin modified resin. 75 cc. heptane To make a negative working developer 0.025 cc. of the following mixture was added to 100x12 cc. of the above mixture:

1 cc. lecithin 158 cc. Freon TF, CCl FCClF The coated support was removed from the developer and the vehicle allowed to rapidly evaporate leaving the pigment toner on the coated support. With a negative corona charge and a positive toner, a positive image of the master was obtained. With a negative corona charge and a negative toner, a negative image was obtained.

The developed image was fixed by applying a dilute solution of resin in Freon TF. Any clear resin soluble in solvents not too active on the .base coating would be satisfactory. Fixing also can be obtained by heating to a suitable temperature to soften the resin in the developer toner or the resin in the coating. Fixing can also be accomplished by means of solvent vapors, transparent pressure-sensitive adhesive sheets or other means to immobilize the toner on the coating.

Example 2 modified casein 80 Zinc oxide 240 Solvent 320 Ammonia, defoamers, orv wetting agents may be added.

The composition was prepared in a pebble mill and coated on a conductive base such as an etched aluminum plate.

The dry coating was sensitized by negative corona discharge (4000 to 8000 volts). Exposure through a negative to actinic light formed a latent electrostatic image capable of development by liquid developer. After developing with liquid developer as in Example 1 and fixing, the plate was put on an offset printing press and used with conventional olfset fountain solution and inks to produce offset copies.

The polarity of the toner particles was changed by the addition of lecithin as described above. This change of polarity of the toner to a negative charge provided a method for obtaining negative copies of the master and, therefore, the electrophotographic process of the present invention provides a means and method for making positive copies and means and method for making negative copies from the same master.

From the disclosure of the present invention it will be apparent that positive and negative images can be produced from a given master by the selection of a suitable toner. For example, the phenol formaldehyde toner with carbon black produces a positive copy while negative images can be made by the addition of lecithin.

It will thus be seen that applicant has provided a safe and effective method of making electrophotographic images by the use of a liquid developer and that the images can be fixed in a number of ways.

What is claimed is:

1. A method of developing a negative electrostatic latent image on an electrophotographic material resistant to trichlorotrifiuoroethane comprising a conductive support and a layer coated on said support, said layer comprising photoconductive zinc oxide and a binder selected from the group consisting of a polymer of esters of acrylic and methacrylic acids, polyvinyl acetate and modified casein, said method comprising the steps of: treating said material with a liquid developer comprising trichlorotrifluoroethane, rosin-modified phenol-formaldehyde resin and finely divided carbon black which is attracted imagewise to said coating and adheres thereto, and allowing the trichlorotrifluoroethane to evaporate and deposit the carbon black imagewise on said layer.

2. The invention according to claim 1 in which the toner is fixed by heat to cause permanent adhesion to the coating.

3. The invention according to claim 1 in which the image is fixed by solvent vapors.

4. The invention according to claim 1 in which the image is fixed by application of a transparent pressuresensitive adhesive sheet thereover.

5. A method in accordance with claim 1 in which said trichlorotrifiuoroethane is 1,l-dichloro-1-fiuoro-2,2-difluoro-Z-chloroethane.

6. A method in accordance with claim 1 in which said developer includes lecithin.

7. A method in accordance with claim 1 in which the electrophotographic material comprising the coating with the developed image is suitable for use as a printing plate.

8. A method in accordance with claim 6 in which the electrophotographic material comprising the coating with the developed image is suitable for use as a printing plate.

References Cited by the Examiner UNITED STATES PATENTS 2,760,431 8/1956 Beatty 96-33 X 2,899,335 8/1959 Straughan 96-1 X 2,954,291 9/1960 Clark 96-1 2,995,085 8/1961 Walkup 96-1 X 3,010,842 6/1962 Ricker 96-1 3,038,799 6/1962 Metcalfe et a1. 96-1 3,058,914 10/ 1962 Metcalfe et a1 117-37 X 3,076,722 2/ 1963 Greig 252-621 3,113,022 12/1963 Cassiers et a1. 96-1 3,150,976 9/1964 Johnson 252-62.l 3,155,503 11/ 1964 Cassiers et a1. 96-1 FOREIGN PATENTS 565,006 10/1958 Canada.

OTHER REFERENCES Chemical Abstracts, vol. 44, column 9499, 1950, and vol. 27, page 4464, 1933.

Chemical Abstracts, vol. 46, column 3770, 1952.

NORMAN G. TORCHIN, Primary Examiner. 

1. A METHOD OF DEVELOPING A NEGATIVE ELECTROSTATIC LATENT IMAGE ON AN ELECTROPHOTOGRAPHIC MATERIAL RESISTANT TO TRICHLOROTRIFLUOROETHANE COMPRISING A CONDUCTIVE SUPPORT AND A LAYER COATED ON SAID SUPPORT, SAID LAYER COMPRISING PHOTOCONDUCTIVE ZINC OXIDE AND A BINDERR SELECTED FROM THE GROUP CONSISTING OF A POLYMER OF ESTERS OF ACRYLIC AND METHACRYLIC ACIDS, POLYVINYL ACETATE AND MODIFIED CASEIN, SAID METHOD COMPRISING THE STEPS OF: TREATING SAID MATERIAL WITH A LIQUID DEVELOPER COMPRISING TRICHLOROTRIFLUOROETHANE, ROSIN-MODIFIED PHENOL-FORMALDEHYDE RESIN AND FINELY DIVIDED CARBON BLACK WHICH IS ATTACHED IMAGEWISE TO SAID COATING AND ADHERES THERETO, AND ALLOWING THE TRICHLOROTRIFLUOROETHANE TO EVAPORATE AND DEPOSIT THE CARBON BLACK IMAGEWISE ON SAID LAYER. 